1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to the use of photopatternable planarization materials, and associated solvents, in the fabrication, of devices made with organic semiconductor (OSC) films.
2. Description of the Related Art
Organic field-effect transistors (OFETs) and integrated circuits can be prepared by means of mass-printing or thin film deposition methods. The selection of print methods for the different layers is determined by dimensional requirements and the properties of printed materials, as well as economic and technical considerations of the final printed products. Optimal resolution of these considerations typically results in a combination of several print methods for the fabrications of the devices, as opposed to a single method.
An OFET is a transistor that uses an organic semiconductor (OSC) in its channel. OTFTs are a type of OFET. OTFTs can be prepared either by vacuum evaporation of small molecules, by solution-casting of polymers or small molecules, or by mechanical transfer of a peeled single-crystalline organic layer onto a substrate. These devices have been developed to realize low-cost, large-area electronic products. OTFTs have been fabricated with various device geometries.
Organic polymers, such as poly(methyl-methacrylate) (PMMA), CYTOP, PVA, polystyrene, parylene, etc., can be used as a dielectric. OFETs employing numerous aromatic and conjugated materials as the active semiconducting layer have been reported, including small molecules such as rubrene, tetracene, pentacene, diindenoperylene, perylenediimides, tetracyanoquinodimethane (TCNQ), and polymers such as polythiophenes (especially poly 3-hexylthiophene (P3HT)), polyfluorene, polydiacetylene, poly 2,5-thienylene vinylene, poly p-phenylene vinylene (PPV). These can be deposited via vacuum or solution base methods, the latter being of interest for printed electronics. The newer generation of solution processable organic semiconductors consists of blends of high performance small molecule and polymeric molecules for optimum performance and uniformity.
As part of a typical IC fabrication process, each level of the IC is finished with a planarization layer that acts as a foundation for an overlying IC level. Ideally, this planarization layer can be patterned using photolithographic processes to form vias that make electrical interconnections between levels. In short, photolithography uses light to transfer a geometric pattern from a photomask to a light-sensitive material. A series of chemical treatments then either engraves the exposure pattern into, or enables deposition of a new material in the desired pattern upon, the material underneath the light-sensitive material. The photolithography process can create extremely small patterns (e.g. nanometers in size), and it affords exact control over the shape and size of the objects it creates because it can create patterns over an entire surface cost-effectively.
Conventional photolithography processes may use a wet chemical treatment, e.g. the so-called RCA clean procedure based on solutions containing hydrogen peroxide, to remove organic or inorganic contaminations. Obviously, this can be an issue when working with organic semiconductors. Further, heat may be used to drive off any accumulated moisture, which may affect film adhesion. After exposure to light a developer is applied and a post-bake performed. The use of developer solvents can be detrimental to OSC films. Further, it is known that OSC be materials can be damaged at temperatures greater than 120° C. Most commercially available photopatternable planarization materials are polyimides with relatively high cure temperatures of greater than 200° C. Fluoropolymer is known to be an effective passivation material to protect OSC film from solvents. However, additional problems exist in creating adhesion between a fluoropolymer passivation layer and an overlying photopatternable planarization layer.
It would be advantageous if a photopatternable planarization layer could be used in the fabrication of OSC transistors.
It would be advantageous if the above-mentioned photopatternable planarization layer could be used in conjunction with a fluoropolymer passivation material.